Protect the free of base->sig.sh_old . This was raising a warning under some finalization circumstances.

This should be moved to v1.3

Thanks to Yaakoub El Khamra for the patch. 

This commit was SVN r21079.

opal/event/signal.c

@@ -301,5 +301,8 @@ evsignal_dealloc(struct event_base *base)
 	base->sig.sh_old_max = 0;
 
 	/* per index frees are handled in evsignal_del() */
-	free(base->sig.sh_old);
+    if( NULL != base->sig.sh_old ) {
+        free(base->sig.sh_old);
+        base->sig.sh_old = NULL;
+    }
 }